The present invention relates to thermistors generally, and more particularly to multilayer thermistors which have a positive temperature coefficient of resistance (PTCR) combined with a room temperature resistance which is lower than that of previous thermistors of the same size.
PTCR thermistors are often used as thermal switches to protect devices from over-heating. They are connected in series with these devices, and when the temperature rises the thermistor's resistance also rises, cutting the flow of current to the device, thereby preventing it from heating up further. It is desirable for the thermistors to have a low resistance at room temperature, so as to interefere as little as possible with the flow of current to the device when it needs no protection.
Generally, thermistors are made in a wafer or disk shape consisting of PTCR material sandwiched between two electrodes. The PTCR material commonly used is barium titanate. It is possible to lower such a thermistor's room temperature resistivity by changing the composition of the barium titanate, but it cannot be lowered indefinitely without degrading the PTCR effect.